1. Field of the Invention
This invention relates to an electron-emitting device that is free from degradation due to long use and the undesired phenomenon of electric discharge under a voltage applied thereto and can emit electrons stably and efficiently for a long time. It also relates to an electron source and an image forming apparatus such as a display apparatus or an exposure apparatus comprising such devices as well as a method of manufacturing the same.
2. Related Background Art
There have been known two types of electron-emitting device; the thermionic cathode type and the cold cathode type. Of these, the cold-cathode emission type refers to devices including field emission type (hereinafter referred to as the FE type) devices, metal/insulation layer/metal type (hereinafter referred to as the MIM type) electron-emitting devices and surface conduction electron-emitting devices. Examples of FE type device include those proposed by W. P. Dyke and W. W. Dolan, xe2x80x9cField emissionxe2x80x9d, Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, xe2x80x9cPHYSICAL Properties of thin-film field emission cathodes with molybdenum conesxe2x80x9d, J. Appl. Phys., 47, 5284 (1976).
Examples of MIM devices are disclosed in papers including C. A. Mead, xe2x80x9cThe tunnel-emission amplifierxe2x80x9d, J. Appl. Phys. a, 32, 646 (1961).
Examples of surface conduction electron-emitting devices include one proposed by M. I. Elinson, Radio Eng. Electron Phys., 10 (1965).
A surface conduction electron-emitting device is realized by utilizing the phenomenon that electrons are emitted out of a small thin film formed on a substrate when an electric current is forced to flow in parallel with the film surface. While Elinson proposes the use of an SnO2 thin film for a device of this type, the use of an Au thin film is proposed in G. Dittmer: xe2x80x9cThin Solid Filmsxe2x80x9d, 9, 317 (1972), whereas the use of In2O3/SnO2 and of carbon thin film is discussed respectively in [M. Hartwell and C. G. Fonstad: xe2x80x9cIEEE Trans. ED Conf.xe2x80x9d, 519 (1975)] and [H. Araki et al.: xe2x80x9cVacuumxe2x80x9d, Vol. 26, No. 1, p. 22 (1983).
FIG. 33 of the accompanying drawings schematically illustrates a typical surface conduction electron-emitting device proposed by M. Hartwell. In FIG. 33, reference numeral 1 denotes a substrate. Reference numeral 4 denotes an electroconductive thin film normally prepared by producing an H-shaped thin metal oxide film by means of sputtering, part of which eventually makes an electron-emitting region 5 when it is subjected to an electrically energizing process referred to as xe2x80x9cenergization formingxe2x80x9d as described hereinafter. In FIG. 33, the thin horizontal area of the metal oxide film separating a pair of device electrodes has a length L of 0.5 to 1 mm and a width W of 0.1 mm.
Conventionally, an electron emitting region 5 is produced in a surface conduction electron-emitting device by subjecting the electroconductive thin film 4 of the device to an electrically energizing preliminary process, which is referred to as xe2x80x9cenergization formingxe2x80x9d. In the energization forming process, a constant DC voltage or a slowly rising DC voltage that rises typically at a rate of 1V/min. is applied to given opposite ends of the electroconductive thin film 4 to partly destroy, deform or transform the film and produce an electron-emitting region 5 which is electrically highly resistive. Thus, the electron-emitting region 5 is part of the electroconductive thin film 4 that typically contains a gap or gaps therein so that electrons may be emitted from the gap.
After the energization forming process, the electron-emitting device is subjected to an xe2x80x9cactivationxe2x80x9d process, where a film (carbon film) of carbon and/or one or more than one carbon compounds is formed in the vicinity of the gap of the electron source in order to improve the electron-emitting performance of the device. The process is normally carried out by applying a pulse voltage to the device in an atmosphere that contains one or more than one organic substances so that carbon and/or one or more than one carbon compounds may be deposited in the vicinity of the electron-emitting region. Note that a deposited carbon film is found mainly on the anode side of the electroconductive thin film and only poorly, if any, on the cathode side. In some cases, a xe2x80x9cstabilizationxe2x80x9d process may be carried out on the electron-emitting device in order to prevent carbon and/or one or more than one carbon compounds from being excessively deposited and the device may show a stabilized performance in the operation of electron emission. In the stabilization process, any organic substances that have been adsorbed in the peripheral areas of the device and those that are remaining in the atmosphere are removed.
For a surface conduction electron-emitting device to operate satisfactorily in practical applications, it has to meet a number of requirements including that it needs to show a large emission current Ie and a high electron emission efficiency xcex7 (=Ie/If, where If is the current that flows between the two device electrodes, which is referred to as device current), that it must operate stably for electron emission after a long use and that no electric discharge phenomenon should be observed on it if a voltage is applied to the device (between the two device electrodes and between the device and an anode).
While the performance of an electron-emitting device is affected by a number of factors, the inventors of the present invention has discovered that the performance is strongly correlated with the shape and the distribution of the carbon film formed on the electron-emitting gap and its vicinity in the activation process as well as the conditions under which the activation process is carried out.
It is, therefore, the object of the present invention to provide an electron-emitting device that performs well for electron emission by selecting optimal conditions for the carbon film in terms of its distribution, its properties and the conditions under which it is treated before producing the device as a finished product.
According to the invention, the above object is achieved by providing an electron-emitting device comprising a carbon film which is made of graphite and formed inside the gap of the electron-emitting region as shown in FIGS. 1A and 1B of the accompanying drawings. While the device of FIGS. 1A and 1B does not practically carry any carbon film outside the gap, a carbon film may also be formed outside the gap. Although graphite is a crystalline substance containing only carbon atoms, its crystallinity may be accompanied, to certain extent, by xe2x80x9cdistortionsxe2x80x9d of various types. For the purpose of the invention, however, a carbon film of highly crystalline graphite is formed in the inside of the gap of the electron-emitting region.
According to an aspect of the invention, there is provided an electron-emitting device comprising a pair of electrodes and an electroconductive film arranged between the electrodes and including an electron-emitting region, characterized in that said electron-emitting region carries a graphite film that shows, in a Raman spectroscopic analysis using a laser light source with a wavelength of 514.5 nm and a spot diameter of 1 um, peaks of scattered light, of which 1) a peak (P2) located in the vicinity of 1,580 cmxe2x88x921 is greater than a peak (P1) located in the vicinity of 1,335 cmxe2x88x921 or 2) the half-width of a peak (P1) located in the vicinity of 1,335 cmxe2x88x921 is not greater than 150 cmxe2x88x921.
According to another aspect of the invention, there is provided a method of manufacturing an electron-emitting device comprising a pair of electrodes and an electroconductive film arranged between the electrodes and including an electron-emitting region, characterized in that it comprises a step of applying a voltage to the electroconductive film containing a gap therein and said voltage is a bipolar pulse voltage.
According to a still another aspect of the invention, there is provided a method of manufacturing an electron-emitting device comprising a pair of electrodes and an-electroconductive film arranged between the electrodes and including an electron-emitting region, characterized in that it comprises a steps of applying a voltage to the electroconductive film containing a gap therein in an atmosphere containing one or more than one organic substances and applying a voltage to the electroconductive film in an atmosphere containing a gas having a composition expressed by XY (where X and Y respectively represent a hydrogen atom and a halogen atom).
According to a still another aspect of the invention, there is provided a method of manufacturing an electron-emitting device comprising a pair of electrodes and an electroconductive film arranged between the electrodes and including an electron-emitting region, characterized in that it comprises steps of forming a graphite film on the electroconductive film including a gap and removing any deposits other than said graphite.